This invention relates in general to optical pressure sensors, and in particular to a pressure sensor capable of measuring small changes in pressure in a high pressure environment, such as oil well logging.
Typically, pressures are measured in oil wells with various types of electronic equipment. The electronic sensors are sensitive to temperature changes and are slow to recover to equilibrium. In addition, the electronic sensors do not have good long-term stability, and hence they do not have good reliability. Pressure sensors are used in oil wells, particularly in oil wells below the ocean floor to determine the opportune time to pump oil from these reservoirs. Because of the severe environmental conditions which exist in these wells, an extremely stable and reliable pressure sensor is required. The pressure sensors generally have to operate up to a pressure of 10,000 psi, with a resolution of 10 psi and must function at a depth of 15,000 feet below the ocean floor. The ambient temperature at which these pressure sensors must operate is of the order of 200 degrees centigrade.
An optical wave is characterized by its amplitude, phase, and wavelength, and all of these parameters can be used for sensing pressure. A number of optical pressure sensing devices are known in the prior art. Pressure sensors based on the intensity of the light have a limited dynamic range, particularly in a high loss system, and elaborate precautions must be taken to insure that spurious losses in the system are not interpreted as changes in pressure. Interferometric devices have virtually unlimited dynamic range and can measure exceedingly small changes in pressure but are difficult to use in determining absolute pressure. The latter is aggravated by the high susceptibility of such devices to fluctuations in temperature. For example, in a typical optical fiber, the phase change induced by a change in temperature of one degree centigrade exceeds the phase change induced by a change in pressure of 1 psi by a factor of about 200. This same temperature to pressure ratio can exceed several thousand in the case of optical pressure sensors which rely on the change in wavelength for pressure sensing. Therefore, in high pressure environments, the conventional optical pressure sensors utilizing changes in intensity, phase, or wavelength, will not be suitable, especially in a high temperature environment.
In U.S. Pat. No. 4,321,831, issued to Tomlinson et al., an apparatus is disclosed for pressure measurement in which a broad band spectrum of light is polarized and then transmitted through a birefringent plate to a pressure sensitive element, such as lucite. The intensity of the light transmitted through the pressure sensitive element varies as a function of wavelength to form a minimum corresponding to a predetermined pressure. The light passes through a second polarizer and then is transmitted to a dispersing means to provide for a digital output in which the wavelength of the output intensity is dependent on the pressure to be monitored.
The device of the Tomlinson, et al. patent requires the use of dual fiber-optic transmission lines to transmit the light to and from the sensor. In addition, the disclosed apparatus identifies a minimum or a single dip in the wavelength spectrum of the transmitted light.
The relevance of the prior art indicated in the present specification should not be given a limited interpretation. A cited prior art item may be found to have relevance in a passage other than the one referred to or to have relevance in a sense different than as stated.